Field of the Invention
This invention relates to a variable speed marine propulsion system and more particularly to a marine propulsion system which is adapted to provide a wide speed range while utilizing a unidirectional prime mover, an alternating current electrical generator, a synchronous electrical motor and a fixed pitch propeller.
Electrical marine propulsion systems provide many advantages not available in mechanical drive systems. They allow control from a number of locations aboard the ship, giving the navigator or operator direct control with a corresponding improvement in responsiveness to changing circumstances experienced during maneuvering operations. Electrical propulsion systems also provide a freedom of installation arrangement not possible in mechanical systems which require an in-line layout between the prime mover and the propeller. The engine-generator set can be positioned anywhere aboard the ship, allowing for greater freedom in distributing other equipment and the drive motor can be located proximate the stern to reduce shaft length.
Since some prime movers such as gas turbines and high-speed diesels are unidirectional machines, electrical propulsion systems possess the added advantages of permitting reverse rotation of the propeller by relatively simple control means and providing the necessary speed reduction to allow the selection of low-speed motors that match the desired propeller speed without the need of mechanical speed-reducing or reversing means.
Electric propulsion systems are classified as either direct-current or alternating-current systems. Direct-current systems have been used on the greatest number and variety of installations. They find application primarily in the low and moderate power ranges (1000 to 6000 horse power per shaft) and where flexibility of setup and ease of control are important whereas the use of alternating-current drives is generally associated with the use of turbine prime movers.
Of the alternating-current electric propulsion systems, early installations used induction motors because their torque performance afforded the conservatism required in applications where little was known about the actual torque requirements of a propeller in maneuvering situations. Also, on warships where the cruising power requirements were low and where an alternate speed ratio contributed to better economy of prime mover operation, the induction motor made practical the arrangement of pole-changing windings to obtain two different speed ratios between the prime mover and the propeller. The more desirable synchronous motors were applied with significant success after experience was gained through development and use of induction motor drive systems.
The use of synchronous motors has some significant advantages as compared to induction motors. Efficiency is improved since a typical slow-speed synchronous propulsion motor has a full-load efficiency of approximately 98 percent as compared to the 94 percent of a comparable induction motor. Since a synchronous motor can be operated at 100 percent power factor as compared to the 70-75 percent of a slow-speed induction motor, savings will be realized in both the weight and cost of the alternating-current generator and the larger air gap of a synchronous motor is conducive to more satisfactory installation and maintenance.
In a typical electrical propulsion system utilizing an alternating-current drive motor and a turbine prime mover, the generator is of the high speed turbo-type and is directly connected to the turbine through appropriate gearing means. The synchronous motor is generally connected directly to the propeller and is of the salient pole type. This type of motor is characterized by its large number of poles that allow it to operate at low speeds while being driven by a generator that operates at 3600 RPM (1800 RPM for four pole generators) and is usually large in diameter and short in length. With synchronous motors, the ratio of speed between the turbine-generator and the propeller motor is fixed by the ratio of the number of poles on the motor and the number of poles on the generator. This characteristic provides the same result as a mechanical reduction gearbox and allows the propeller speed to be controlled by regulating the fuel supply, and therefore the speed, of the prime mover. Since the motor must be capable of operating as in induction motor under the heavy torque loadings experienced during reversing conditions with headway on the ship, it must also be provided with a heavy pole face winding.
When starting or reversing the motor, it must operate as an induction motor until its speed is electrically close enough to that of the generator so that it may be synchronized. During this out-of-synchronization operation the motor power factor is low with high current demands. In order that generator voltage be maintained and the current needed to develop proper motor torque be provided, the generator must be over-excited on a short term basis. When the motor is reversed from full speed ahead, the continuation of the ship's movement through the water causes the water to flow past the propeller, resisting the motor's effort to stop and reverse and imposing severe conditions on it.
Speed control of the motor is typically accomplished by altering the frequency of the alternating current obtained from the generator. This is done by varying the speed of the turbine generator set. The turbine is usually under the control of a governing system with a working range of from 20 percent to 100 percent of maximum speed. All steady-state running is performed with the motor synchronized to the generator with speeds proportional to each other, whereas starting and reversing the motor requires asynchronous operation and the corresponding induction motor operational capability.
An alternating-current generator feeding a synchronous motor is the most economical electrical propulsion system to cover a wide range of motor speeds with a variable speed prime mover. However, in order to provide sufficient maneuvering torque for propeller reversal capability the ship propulsion design must specify larger motors to have sufficient induction motor torque. This is due to the fact that low-speed induction motors are inherently larger than synchronous motors.
The superior maneuvering capability of electric propulsion systems is of a great advantage in ice breakers. Ice breakers subject their propulsion systems to severe conditions during ice breaking operations. When the propeller collides with an ice formation, a geared motor would transfer the impact through the entire propulsion system to the prime mover, but an electric propulsion system effectively decouples the propeller from the prime mover and damage to the prime mover is avoided. Another ice breaking situation where electric propulsion systems are advantageous is when the propeller is frozen in the ice formation. To force it, a high torque is required at very low speed. Electric systems allow this to be accomplished. Generally, when the ship is involved in ice breaking operations, its movement comprises a series of collisions with the ice. If the ice proves too solid to be broken by the ship's present speed, the ice breaker backs up and collides again at a higher speed. This frequent changing of speed and direction demands the maneuverability made possible by an electric propulsion system.
A number of control systems which are known to those skilled in the art have been used to facilitate ship maneuvering. One such system, employing an alternating-current generator, synchronous motor and fixed pitch propeller, is shown in U.S. Pat. No. 3,993,912 of T. E. Eckstrom et al., issued Nov. 23, 1976. However, the Eckstrom system also uses synchronous motors in a way that requires them to have significant induction motor capabilities.
Future expansion of the use of electric ship propulsion systems that utilize synchronous motors will depend on the size and cost reduction of these systems without a corresponding sacrifice of the capability to perform sudden stops through the use of propeller reversal techniques. These reductions will be greatly facilitated if the requirement that the motor be able to perform as an induction motor can be minimized. The primary objective of the present invention, then, is to provide a marine propulsion system that utilizes a unidirectional prime mover, an alternating-current electrical generator and a synchronous motor but which reduces the requirement that the synchronous motor be able to perform as an induction motor. Since this induction motor capability increases the size requirements of the motor and therefore its cost, its minimization will make possible the use of smaller and less expensive motors and expand the application of alternating current motors in marine propulsion systems.